Since the emergence of a resist for a KrF excimer laser (248 nm), it has been of common practice to, in order to compensate for any sensitivity deterioration caused by light absorption, employ an image forming method through chemical amplification as a resist image forming method. A brief description of a positive image forming method through chemical amplification is given below by way of example. Upon exposure, an acid generator is decomposed at exposed areas to thereby generate an acid. In the post-exposure bake (PEB: Post-Exposure Bake), the generated acid is used as a reaction catalyst so that an alkali-insoluble group is converted to an alkali-soluble group. Thereafter, alkali development is carried out to thereby remove the exposed areas. Thus, the relevant image forming method is provided.
In accordance with the miniaturization of semiconductor elements, the wavelength shortening of the exposure light source and the realization of high numerical apertures (high NA) for projector lenses have been advanced. At present, an exposure unit using an ArF excimer laser of 193 nm wavelength as a light source is available. As is commonly known, the following formulae can be established therefor.(Resolving power)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 
In the formulae, λ is the wavelength of the exposure light source; NA is the numerical aperture of the projector lens; and k1 and k2 are factors relating to the process.
As a technology for enhancing the resolving power, it is heretofore known to employ a liquid immersion technique, that is, a method in which the space between a projector lens and a sample is filled with a liquid of high refractive index (hereinafter also referred to as a “liquid for liquid immersion”).
The “effect of the liquid immersion” is as follows. Taking λ0 as the wavelength of exposure light in air, n as the refractive index of the liquid for liquid immersion to air and θ as the convergent half angle of the light beam, where NA0=sin θ, the above-mentioned resolving power and focal depth in the event of liquid immersion can be expressed by the following formulae.(Resolving power)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 
That is, the effect of the liquid immersion is equivalent to the use of an exposure wavelength of 1/n. In other words, in projection optical systems of identical NA, liquid immersion enables the focal depth to be n-fold. This is effective in all pattern configurations. Further, this can be combined with a super-resolution technology, such as a phase shift method or a modified illumination method, now under study.
As techniques for further enhancing the resolving power, it has been proposed to employ a double exposure technique or a double patterning technique. These both lower the value of k1 in the above formula of resolving power. Such techniques enhance the resolving power.
In the conventional patterning for an electronic device, such as a semiconductor element, a reduced transfer of the pattern of a mask or rectile whose pattern size corresponds to 4 to 5 times magnification of that desired onto an exposure subject, such as a wafer, has been carried out by means of a reduced projection exposure unit.
However, in accordance with the miniaturization of the pattern size, the conventional exposure system poses the problem that light beams having been irradiated to adjacent patterns interfere with each other, which reduces the optical contrast. Accordingly, in the conventional technology, an exposure mask design is divided into two or more sections, and individual masks are independently exposed and the resultant images are synthesized. In this double exposure system, it is required to divide an exposure mask design and carry out a re-synthesis of images for the design on an exposure subject (wafer). Thus, it is needed to devise how to divide the mask design so that the pattern on the rectile can be faithfully reproduced on the exposure subject.
Patent reference 1 provides a study of the effect of this double exposure system on the transfer of a microscopic image pattern of a semiconductor element.
However, the patterning by simple application of the conventional resist composition to the conventional resist processing poses the problem that as patterning must be performed in the vicinity of the resolution limit of the resist in this double exposure system, satisfactory exposure margin and focal depth cannot be obtained.
As for the development in g-ray, i-ray, KrF, ArF, EB or EUV lithography, various developers have been proposed to date, and an aqueous alkali developer containing 2.38 mass % TMAH (tetramethylammonium hydroxide) is commonly used.
However, the current situation is that it is extremely difficult to find an appropriate combination of resist composition, developer, rinse liquid, etc. as required for forming a pattern of comprehensively excellent performance, so that an improvement has been demanded in this field. In particular, in accordance with the enhancement of the fineness of resolved line width of the resist, it is increasingly demanded to improve the line edge roughness performance with respect to line patterns and to improve the in-plane uniformity of a pattern dimension.
Moreover, the conventional combination of resist composition and developer only provides a system in which a specified resist composition is combined with a highly polar alkali developer or a developer containing an organic solvent of low polarity to thereby form a pattern. That is, referring to FIG. 1A, a positive system (combination of resist composition and positive developer) only provides a material capable of selectively dissolving/removing any region of high light irradiation intensity RP within the spatial frequency of an optical image to thereby form a pattern. In contrast, referring to FIG. 1B, a negative system (combination of resist composition and negative developer) only provides a material capable of selectively dissolving/removing any region of low light irradiation intensity RN to thereby form a pattern.
Herein, the expression “positive developer” means a developer capable of selectively dissolving/removing any region RP of exposure not lower than the given threshold value T2 indicated by a full line in FIG. 1A; for example, an alkali developer. The expression “negative developer” means a developer capable of selectively dissolving/removing any region RN of exposure not higher than the given threshold value T1 indicated by a full line in FIG. 1B; for example, a developer containing an organic solvent. The development step using the positive developer is referred to as positive development (also referred to as a positive development step), and the development step using the negative developer is referred to as negative development (also referred to as a negative development step).
A double development technique as a double patterning technology for enhancing the resolving power is described in patent reference 2. In this technique, use is made of a common image forming method through chemical amplification. Using the phenomenon that upon exposure, the polarity of the resin contained in the resist composition becomes high in a region of high light intensity while the polarity of the resin becomes low in a region of low light intensity, the region of high exposure of a specified resist film is dissolved by a developer of high polarity to thereby attain a positive development, and the region of low exposure is dissolved by a developer of low polarity to thereby attain a negative development. For example, referring to FIG. 2, the region RP of exposure not lower than the threshold value T2 by irradiation light 1 is dissolved by use of an aqueous alkali solution as a positive developer, and the region RN of exposure not higher than the threshold value T1 is dissolved by use of a specified organic solvent as a negative developer. Accordingly, as shown in FIG. 2, the region of intermediate exposure amount (E2−E1) remains undeveloped, so that a pattern 3 of L/S with a half pitch of exposure mask 2 is formed on a wafer 4.
However, it is extremely difficult to select the most appropriate combination of resist composition and negative developer. The above-mentioned technique encounters the problem that the developability upon the use of the negative developer is deteriorated.
Further, in the formation of a micropattern by double development, it is unsatisfactory to only ensure a high resolving power upon the independent use of either a negative developer or a positive developer. It is demanded to exhibit an excellent pattern resolution in both a negative developer and a positive developer.
Taking the above problems into account, a patterning method using, in the double development technique, a positive resist composition that when exposed to actinic rays or radiation, has its solubility in a positive developer increased and has its solubility in a negative developer decreased is proposed in patent reference 3. It is described that a high-precision micropattern can be stably obtained by this technology.
However, it is still demanded to stably obtain a higher-precision micropattern excelling in line width roughness (LWR) and focus latitude (depth of focus DOF).    [Patent reference 1] Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 2006-156422,    [Patent reference 2] JP-A-2000-199953, and    [Patent reference 3] US Patent Application 2008/0187860 A.